The present invention relates to electromagnetic interference (EMI) shielding, and more specifically, to an improved composite polymeric material for providing enhanced EMI shielding.
An electromagnetic signal is propagated with an electrical field component and a magnetic field component. The electromagnetic signal can interfere with the propagating signal in an electronic circuit assembly resulting in electromagnetic interference. Electromagnetic interference is the generation of undesired electrical signals in electronic system circuitry because of the unintentional coupling of impinging electromagnetic field energy.
Enormous progress in nanotechnology has made electronic systems smaller and has increased the density of electrical components within an instrument. The operating frequency of signals in these systems is also increasing by the day. The increasing operating frequency in these electrical systems results in an elevated level of high frequency electromagnetic interference (EMI). The prevalence of high frequency systems and portable electronic circuitries are creating a very complex spectral environment for the operation of sensitive electrical/electronic systems. Accordingly, it is often advantageous to shield an electrical/electronic circuit assembly to prevent it from emitting EMI, or to shield an electrical/electronic circuit assembly from EMI emitted elsewhere.
The electromagnetic interference shielding of electronic component assemblies has taken many forms. Sensitive or radiating devices may be covered with a lid and/or an enclosure which is connected to ground potential in the process of securing the cover in place. Shielding close to the source, where the field intensity is the highest, requires greater shield efficiency to contain the field.
It is common to shield the sensitive, electromagnetic interference receiving component or even the entire circuit board using conductive composites. Conducting polymer materials, such as a screen-printable copper filled epoxy paste, are generally used to form a shield.
Conductive composites, in the form of coatings, strips or molded materials, can be prepared by the addition of highly conductive metal fillers or powders to non-conductive polymer substrates. There is frequently a tradeoff of electrical performance and physical characteristics desired in a selected application. Increasing the conductivity of the composite, for example by adding conductive metal powder, leads to a reduction in the physical performance of the composite. On the other hand, if the amount of conductive metal powder added to the composite is reduced, the physical performance is enhanced but the electrical performance deteriorates.
Typical coating compositions include electrically-conductive paints, conductively-filled molded elastomeric layers, or flame-sprayed or other deposited metal layers. A conductive gasket may be used to provide electrical continuity between the coating-layers applied to the various mating housing parts.
Various enclosed systems are known to a person of ordinary skill in the art. The enclosed systems are powered by external alternating current and are shielded from electromagnetic interference by the incorporation of internal shields which are connected to a ground potential. A metal cabinet housing which encloses the system may be designed to function as a shield. However, metal housings are often expensive, heavy, and difficult to make in complex shapes. The inside of a molded plastic housing may be coated with a thin metal film through vacuum metallization, but this process often yields a brittle and less flexible shield. Another method is to coat the enclosure with a thin film of a conductor using a metal-filled paint. A metal-filled plastic may also be used to form the housing.
Existing shield assemblies provide limited reuse capabilities and are limited to single compartments. The materials such as paint or plate injection molds or thermoformed materials have not been successfully deployed due to low shielding capabilities. A form-in place gasket has been tried in the existing assemblies, but not in conjunction with a thermally sprayed coating and an electrically conductive plastic.
It is therefore desirable to provide a polymeric material which, when molded with appropriate conductive fillers and shaped into an EMI shielding substrate or component, exhibits improved EMI shielding properties.
Accordingly, it is an object of the invention to provide an EMI shielding material that has improved shielding effectiveness.
It is another object of the invention to provide an EMI shielding material that has improved conductivity.
It is another object of the invention to provide an EMI shielding material that has high tensile strength and flexural strength.